MAKING ACCURATE WEIGHT MEASUREMENTS

The Right Scale For The Job

Scales are used everyday throughout the world in a wide range of industries. From the raw material used in the products around us, to the food that we buy at the grocery store, almost everything around us was weighed using a scale at some point.

Like most other measuring devices, digital scales come in a wide range of capacities and accuracy classes. For instance, most deli scales can weigh quantities up to 50 lbs or more, but are accurate to only a few grams (±3g). On the other hand, a diamond or jewelry scale might be accurate to a few milligrams (±0.003g), but have a maximum capacity of 100g. For food portion use, a deli scale measures weight accurately enough to make sandwiches, but if you had to measure something like gold or platinum, you would want to use a jewelry scale. Choosing the right scale is crucial for success.

Measurement Accuracy

But how do you know if the scale you are using is accurate? If you check the various clocks around you, you might notice they don’t all agree with each other, with some more accurate than others. The process of checking a measuring devices accuracy is called calibration. Calibration is done by comparing your measurements with a reference standard. For your clocks, you would compare the time with one of the atomic clocks using a service such as the NIST’s time and date server time.gov. For digital scales, you have to use accurate calibration weights that match your scale’s specific requirements.

All of our digital scales come calibrated from the factory and most users will not need to calibrate their scale for some time. However, if you require the most accuracy from your scale, you should purchase a calibration weight along with your scale to check accuracy and adjust as necessary. Calibration and adjustment of your scale is permanent so you should make sure you are using the correct mass listed in your user manual or you risk mis-calibrating your scale.

Common Misconceptions

Because scales are so prevalent, we often take for granted the underlying science and technology that makes them work. There are some common mistakes that new users make when using and calibrating precision scales which can lead to measurement errors.

Many food items have weights and volumes printed on them like water bottles or soup cans. It might seem like these weights would be suitable for checking your scale, but this number is actually the item's “net weight” which means the weight of it's contents, not it's "gross weight", or total weight including the container or packaging. The weights of packaged food items also have certain allowances for error which makes these items not suitable for determining the accuracy of scales.

One of the most prevalent misconceptions with calibration of digital scales involves the US “nickel” or five cent piece. The nickel has a nominal weight of 5.000g as stated on the US Mint’s website, but in circulation nickels can have weights ranging from 4.9g to 5.1g due to corrosion and wear and tear. Although combining nickels to make something approximating the required calibration mass for your scale is possible, this method will produce poor results due to the varying weights of the individual nickels.

If you use the wrong weight to calibrate your scale, you risk seriously impacting the accuracy. Most scales only calibrate with specific mass values. For instance, most 100g capacity scales require a 100g calibration weight to calibrate, while 500g capacity scales typically use a 500g calibration weight. If you placed a 5g nickel (4.9-5.1g) on the scale during calibration when it is asking for 500g, you are telling your scale that the 5g nickel is 500g. Afterwards, your measurements would be weighing off by a factor of 100 (ie. 1g would read as 100g, 2g would read as 200g, etc). This is why it is important to use accurate calibration weights that match the requirements of your scale. Only calibration weights designed for scales are accurate enough to use when performing calibration adjustment.

Sources Of Error

Calibration of your scale is only one part of maintaining accuracy in your measurements. There are also a number of environmental influences that can affect measurement accuracy which you should try to account for and minimize. These include:

Gravity- The differences in local gravity around the world make a difference in how much things weigh. Scales often give readings in units of mass such as grams or ounces, but they are actually measuring weight or the force due to the Earth's gravity pulling the item downward. While the mass of an object remains constant, it’s weight changes depending on the force due to gravity exerted on it. Gravity is stronger the closer your are to an objects center of mass. Since the Earth is roughly a sphere, for most purposes the force of gravity is stated as 1G or 9.8m/s^2 everywhere on the surface. However, due to the planet’s rotation the Earth is actually more like a squished sphere which gets wider towards the Equator. As you get closer to the Equator, you actually weigh less because you get further from the center of the Earth. The local elevation and underlying geography of a location also affect its gravity. With sensitive lab scales, even moving the scale up a few floors in the same building affects accuracy enough to require calibrating the scale again. Calibrating a digital scale at its location of use eliminates the error caused by differences in gravity, insuring that the same mass weighs the same anywhere that scale is used.

Wind- Drafts of wind from nearby vents or fans can be detected by precision scales, even if you don’t necessarily feel them. You should use your scale in a location free from drafts or use a draft shield.

Temperature- The metal load cell inside the scale expands and contracts slightly as temperatures change. These changes are corrected for using temperature compensation sensors built into the weighing circuit, but large fluctuations in temperature will cause scales to drift. If temperatures change drastically, you may need to let your scale adjust to its new surrounding temperature, then perform calibration. Digital scales perform best at normal room temperatures.

Vibration- Sensitive scales can pick up slight vibrations from nearby machinery or foot traffic. You should find a level surface that is free of vibration to use your scale. A heavy counter top or sturdy table should work well. You can also use a mouse pad or anti-vibration mat to help keep your scale stable.

Static- The sensitive circuits inside digital scales can be disturbed by static electricity. Static may cause the weight readings to fluctuate randomly. Try to eliminate static by keeping humidity levels in a comfortable range or use methods such as anti-static wipes to reduce static around your work space.

Radio Frequency Interference (RFI)- Strong radio wave emitting devices can also disturb digital scale readings. You should keep computers, cell phones, and routers away from your scale to minimize interference.

User Proficiency

Last but not least, you should practice good measurement technique when taking weight measurements with your scale. Consistency is key. Be sure to place your items on the center of the weighing platform and do not leave items on the scale for prolonged periods of time. Pressing Tare/Zero between every weighing is recommended. This resets the zero point internally and eliminates any potential errors which can occur from slight mechanical shifts during weighing. You should also try not to breathe over the scale or lean against the table or counter top as this will cause the scale to tilt slightly and affect the readings. When measurements are critical, multiple measurements are sometimes made and averaged.